In order to realize safety securement, soundness maintenance of a fuel, and efficient combustion of the fuel in a nuclear reactor, power distributions at respective parts of a reactor core are monitored by detectors of a neutron instrumentation system. Specifically, the detectors of the neutron instrumentation system such as a source range monitor, an intermediate range monitor, a local range power monitor are dispersed in a reactor core in a state of being housed in in-core instrumentation tubes.
The in-core instrumentation tubes are sequentially changed at the time of regular inspections according to a plan in consideration of life of the detectors of the neutron instrumentation system and irradiation life of the in-core instrumentation tubes themselves.
Here, a nuclear reactor containment vessel of a plant using the traversing in-core probe generally has an access tunnel inside. This access tunnel enables a worker to access a nuclear reactor pressure vessel side from the outside at the time of, for example, the regular inspection of the traversing in-core probe. Further, on a ceiling portion in the access tunnel, indexing devices for guiding a guide tube to the nuclear reactor pressure vessel side is installed.